In recent years, direct sequence (DS) code division multiple access (CDMA) spread spectrum communication systems and methods have experienced growing attention worldwide. The IS-95 cellular communication standard is one example of an application of DS-CDMA communications, as described in the article TIA/EIA/IS-95-A, "Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System", Feb. 27, 1996. Additional applications include third generation cellular systems, wireless multimedia systems, personal satellite mobile systems, and more.
In DS-CDMA communications, each user is assigned a distinct spreading code often referred to as pseudo noise (PN) sequence. The spreading code bits (called chips) are used to modulate the user data. The number of chips used to modulate one data symbol is known as the spreading factor of the system, and it is related to the spreading in bandwidth between the (unmodulated) user data and the CDMA signal. In this simplest form, the base-band equivalent of the transmitted CDMA signal, sampled at the chip rate 1/Tc, is ##EQU1##
where Tc is the chip duration, .left brkt-bot.x.right brkt-bot. denotes the integer part of x, SF is the spreading factor, .alpha..sub.i [.left brkt-bot.n/SF.right brkt-bot.] and PN.sub.i [n] are the data symbol and spreading code of the i-th user, respectively, and K is the number of active users. Note that by the definition of .left brkt-bot.x.right brkt-bot., .alpha..sub.i [.left brkt-bot.n/SF.right brkt-bot.] is fixed for SF consecutive chips, in accordance with the definition above that each data symbol is modulated by SF chips.
An important feature of DS-CDMA systems is that they provide the possibility of obtaining excellent immunity to multipath fading through resolving the individual, time separated multipath components and optimally combining them. The common approach for achieving this is to use a "rake" receiver as it is known in the art. Such a receiver assigns despreading correlators to each of the dominant multipath components and synchronizes them for maximum de-spread power. For each of the rake "fingers", the phase and amplitude of the corresponding channel multipath component is estimated and used to apply amplitude weighting and phase alignment prior to combining. The weighted sum of the multipath components will experience considerably less fading than any of the individual components so that a diversity gain is obtained.
As is known in the art, a crucial requirement of the rake receiver is that its fingers are time aligned (synchronized) with the multipath components of the channel. This requires estimation of the multipath delays and is often achieved by a simple early-late time tracking mechanism. The early-late mechanism is, in fact, a delay-lock-loop that measures the energy prior (early) and after (late) the current sampling instances. These early and late energy measurements are used to lock on the sampling instance that maximizes the sampled signal energy. As it turns out, these maximal energy sampling instances leads, in many cases, to the desired synchronization of the rake fingers to the channel multipath components. However, some channels, for example those encountered in dense urban environments, consist of a large number of closely spaced multipath components. This leads to multipath clusters that are often spaced less than Tc apart. Conventional early-late time tracking mechanism are often incapable of tracking the delays associated with those closely spaced multipath clusters since their early and late measures are a superposition of the energies associated with several adjacent clusters. In such a situation, the rake fingers are not properly time aligned with the multipath clusters, leading to degradation in the receiver performance.
It would therefore be beneficial to have an improved time tracking mechanism that is more robust to the presence of closely spaced multipath components.
It would also be beneficial to have an improved criterion for finger assignment in closely spaced multipath environment.
In recent years several methods for combating closely spaced multipath components in DS-CDMA communication systems were derived. In U.S. Pat. No. 5,692,006 to Ross, U.S. Pat. No. 5,648,983 to Kostic et al. and U.S. Pat. No. 5,793,796 to Hulbert et al. it is suggested to avoid direct estimation of the path delays, Instead, a bank of closely spaced fingers is utilized to effectively cover a pre-specified delay window. Thus instead of actually estimating the multipath delays all possible delays in the window are examined and weighted according to some quality measure criterion. In U.S. Pat. No. 5,692,006 a conventional LMS algorithm is used to adaptively estimate optimal finger weighting, whereas in U.S. Pat. No. 5,648,983 a weighted least squares solution is used to assign the finger weights.
Other solutions can be found in:
EP Patent Publication 704 985 A2 to Hulbert;
U.S. Pat. No. 5,764,688 to Hulbert et al.;
I. Dumont, et al., "Super-resolution of Multipath Channels in a Spread Spectrum Location System", Electronics Letters, Vol. 30, No. 19, Sep. 15, 1984; and
Makoto Takeuchi et al., "A Delay Lock Loop Using Delay Path Cancellation for Mobile Communications", Electronics and Communication in Japan, Part 1, Vol. 79, No. 4, 1996.